Glacial Earthquakes, three years on

Three years ago we reported on the discovery of glacial earthquakes, a new class of earthquakes associated with fast-moving glaciers. These seismic events generate long-period (25--100 sec) Rayleigh and Love waves that in amplitude are equivalent to those generated by a traditional magnitude-5 earthquake, but which produce little high-frequency energy. We have detected a small number of events of this type in Alaska and Antarctica, but more than 95% of the located events (some 200 for the period 1993--2005) are associated with outlet glaciers on Greenland. A strong seasonal signal is evident in the frequency of glacial earthquakes on Greenland, with most events occurring during August and September, coincident with the period of maximum melting. A clear trend in the frequency of events was identified for the period 2001--2005, with each year generating a larger number of events. This trend appears to have been broken in 2006. The seismic waveforms for glacial earthquakes are well explained by a mass-sliding mechanism, and are not consistent with a tectonic stress-release mechanism, leading us to our preferred interpretation that the events are associated with a sudden melt-water-facilitated downhill sliding motion of a large portion of the glacier. Force geometries determined for the events are in general consistent with this interpretation, but ambiguities remain. Alternative explanations, such as large-scale sudden internal deformation, should also be considered, as these events occur where glacial strain rates are very high. Near-field geodetic and seismic observations will be required to further constrain and elucidate the nature of these events, and a deployment of 20 continuously recording GPS instruments on Helheim glacier during the summer of 2006 may have provided new observations to address this phenomenon.